#include <iostream>
#include <fstream>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;
ifstream in("arie.in");
ofstream out("arie.out");

#define ll long long
#define ull unsigned long long
#define pb push_back
const int NMax = 1e5 + 5;

int N;
pair<int,int> v[NMax];

int main() {
    in>>N;
    for (int i=1;i <= N;++i) {
        in>>v[i].first>>v[i].second;
    }

    v[N+1] = v[1];
    v[0] = v[N];
    ll arie = 0;
    for (int i=1;i <= N;++i) {
        arie += 1ll * v[i].first * v[i+1].second;
        arie -= 1ll * v[i].first * v[i-1].second;
    }
    out<<arie/2<<'\n';

    in.close();out.close();
    return 0;
}